Benefits

This is nice, but what are the actual benefits of “bootstrappable” implementations?

For users

As a user, bootstrappable implementations, together with reproducible builds, provide confidence that you are running the code you expect to be running.
Its source code is auditable by the developer community, which in turns provides reassurance that the code you’re running does not have backdoors.

For distributors

Having an automated process for bootstrapping your distribution on existing architectures greatly simplifies porting to new architectures.
With a formalised system bootstrap process you can just focus on issues specific to the new architecture.

For developers

If you are a compiler writer, making your compiler bootstrappable from a different language will simplify the development process (no need to carry large pre-built binaries around).
It will also make it easier to port the compiler to a different platform for which no bootstrap binaries exist yet.